1. Field of the Invention
This invention relates to level sensing devices, and more particularly, to devices for determining the level of the contents within containers while the containers are being filled.
2. Description of the Prior Art
In the food processing industry there has long been a need to accurately and dependably fill containers with the proper amount of food. Most commonly a mechanical sensor is used during filling to measure the level of food within the container. These mechanical sensors usually have a foot that extends down into the container and is counterbalanced to lightly touch the top of the food therein. As the container fills, the contents push the foot upward until the foot and its associated linkage trip a microswitch. The actuation of the microswitch indicates that a predetermined level of food in the container has been reached and terminates the filling operation.
One problem with the mechanical sensors is interference with the filling operation. Since the mechanical foot must extend down into the container, very often the foot and its linkage block the mouth of the container and obstruct the entry of food into the container. In addition, the food frequently piles up on the foot and prevents the foot from following the rising level of the contents in the container. When the rising motion of the foot is blocked by the pile of food thereon, the container is filled to overflowing since the microswitch is never actuated.
The problems involved with the use of a mechanical sensor are even more acute when the containers are to be only partially filled at one processing station. Partial filling is usually done at one processing station in order to pack one or more different types of food within the same container at succeeding stations. For partial filling the foot must extend much further down into the container in order to measure the lowest filling level, and, thus, the foot is much more likely to block the entrance to the container during the filling operation.
While optical level detecting devices have been proposed in the past for checking or determining the level of fill in a container, as shown in the issued U.S. patents to Berthelsen No. 3,267,287 and Walker No. 3,404,282 for example, such devices have had only limited utility since they were designed to operate only with liquid materials providing a level highly reflective surface or they were useful only for checking levels at or near the top of the container.